This invention relates generally to a digital broadcast receiver having a function of controlling the gain of the receiver by feeding back received broadcasting signals to an AGC (Automatic Gain Control) circuit.
In recent years, digital broadcasting signals in accordance with the ATSC (Advanced Television Systems Committee) standard has been transmitted from broadcasting stations. Therefore, broadcast receivers (set top box: abbreviated as STB hereinafter) are being spread which receive the digital broadcasting signals, converting the digital broadcasting signals into analog signals which can be received by existing television sets adapted to analog broadcasting in accordance with the NTSC (National Television Systems Committee) standard, and outputting the analog signals thereto. Since the present is in a period of transition from analog broadcasting to digital broadcasting, both analog broadcasting signals and digital broadcasting signals are transmitted. The STB has, therefore, a configuration including a tuner capable of receiving both the analog and digital broadcasting signals.
The intensity level of an electric wave received by the STB varies according to the intensities of electric waves generated by various broadcasting stations, weather conditions in a propagation path, and the like. Usually, broadcast receivers have therein an AGC circuit for suppressing fluctuations in contrast of an output image by automatically controlling (adjusting) the gain of a tuner in accordance with the intensity of a received electric wave to obtain a constant video detection output. As such an STB having therein an AGC circuit, there is a known broadcast receiver having a tuner for receiving both digital and analog signals, a digital AGC circuit for controlling the gain of the tuner when digital broadcasting signals are received, and an analog AGC circuit for controlling the gain of the tuner when analog broadcasting signals are received (refer to, for example, Japanese Patent Laid-open Application Nos. 11-98426, 11-261922, and 11-341392). In the broadcast receiver, signals outputted from the above-described two AGC circuits are switched by a separately provided voltage converting circuit (switching unit).
As a method of controlling the gain of a digital broadcasting signal, the following two methods have been already invented. According to the first method, broadcasting signals received by a tuner are sent to a filter such as an SAW (Surface Acoustic Wave) filter which allows broadcasting signals in a desired frequency band to pass therethrough. The broadcasting signals passed are A/D converted, demodulated in a predetermined manner, and then detected to be fed back to an AGC circuit. In such a process, the gain control is performed. In this case, the signals passed through the SAW filter are fed back to the AGC circuit, so that the gain control of the tuner is performed on the basis of the intensity level of signals on a channel to be received. According to the second method, the broadcasting signals received by the tuner are directly fed back to the AGC circuit, so that the gain control is performed on the basis of the broadcasting signals. In this case, the signals fed back to the AGC circuit have not passed through the SAW filter. Consequently, the gain control of the tuner is performed on the basis of the intensity levels of broadcasting signals in frequency bands of a channel to be received and of channels in the proximity to the channel,.
However, the intensity level of a digital broadcasting signal is lower than that of an analog broadcasting signal. In a conventional broadcast receiver, consequently, there is such a case that a proper gain control cannot be performed even when the above method is applied. For example, in the case where the digital broadcasting signals are transmitted on 3 CH, the analog broadcasting signals are transmitted on 2 CH and 4 CH adjacent to the 3 CH (state shown in FIG. 2A), and a user selects the 3 CH, if the first method is applied, the following problem occurs. That is, since the AGC circuit controls the gain of the tuner so that the signal intensity level of a channel to be received (e.g., 3 CH) becomes the optimum as described above, distortion may occur in signals on the 2 CH and the 4 CH having the signal intensity level higher than that on the 3 CH. The distortion which occurs in the signals on the 2 CH and the 4 CH exerts an influence on the signals on the 3 CH adjacent to the 2 CH and the 4 CH, and the signals cannot be properly received.
In the case where the digital broadcasting signals are transmitted on the 3 CH, the analog broadcasting signals are transmitted on 1 CH and 5 CH which are adjacent to the 3 CH with idle channels (2 CH and 4 CH) between (state shown in FIG. 2B), and the user selects the above-described 3 CH, if the second method is applied, the following problem occurs. That is, although unnecessary frequency components in the signals to be fed back to the AGC circuit are cut out by a filter provided at the output stage of the tuner, since the frequency characteristic of the filter in the tuner is gentle, the filter cannot separate broadcasting signals on the 1 CH and the 5 CH. Consequently, the broadcasting signals on the channels are fed back to the AGC circuit. As a result, the AGC circuit controls the gain of the tuner so that the range of the 1 CH or the 5 CH becomes the maximum range. Thus received broadcasting signals on the 3 CH has therefore a low intensity level since the gain control is not properly performed. Accordingly, the resolution of multi-value digital signals obtained by A/D converting the above-described broadcasting signals deteriorates, and an error rate at the time of demodulation of the multi-value digital signals to binary digital signals increases.